HISTORY
Question refers to the excerpt below.
"The appeal of the undiscovered is strong in America. For three centuries the fundamental process in its history was the westward movement, the discovery and occupation of the vast free spaces of the continent. We are the first generation of Americans who can look back upon that era as a historic movement now coming to its end. Other generations have been so much a part of it that they could hardly comprehend its significance. To them it seemed inevitable. The free land and the natural resources seemed practically inexhaustible."—Frederick Jackson Turner, from The Frontier in American History, Chapter XIII, 1921
Turner's notion of the "appeal of the undiscovered" to Americans is most clearly seen in which of the following?
Expansion in the Pacific
Border disputes with Canada
Internal conflicts over the Caribbean
Western land purchases